1. Field of the Invention
This invention relates generally to local area networks and more particularly, it relates to a method and apparatus for data transmission utilizing a medium access control (MAC) layer protocol in a slotted unidirectional bus local area network to provide a high efficiency and complete fair cell-access scheme that is independent of the number of network stations connected to the bus.
2. Description of the Prior Art
As is known in the information technology, there exists a number of organizations such as CCITT, TI, IEEE, and ISO that define interfaces, networks, and protocols so that different equipments can communicate with each other. For example, a subnetworking system for use in metropolitan area network (MAN) is the distributed queue dual bus (DQDB) as defined in IEEE 802.6 DQDB Metropolitan Area Networks (proposed) standard. The DQDB subnetwork consists of two oppositely oriented unidirectional buses and a plurality of stations distributed along the length of the buses. For a more detailed description and as a background, reference is made to an article authored by R. M. Newman, Z. L. Budrikis, and J. L. Hullett entitled "The QPSX Man" in IEEE Communications Magazine, Vol. 26, No. 4, April, 1988, pp. 20-28.
The DQDB subnetwork provides two types of cell transmission services: (1) isochronous cell transmission mode that allows connected stations to establish multiple virtual channels for voice and video transmission services requiring steady bandwidth, and (2) asynchronous cell access mode defined under the concept of Asynchronous Transfer Mode (ATM) to provide a high efficiency or burst information transport. See the article entitled "ATM Ring Protocol and Performance" authored by H. Ohnishi, N. Morita, S. Suzuki of NTT Communication Switching Labs of Tokyo, Japan. The bandwidth not reserved for isochronous cell transmissions is shared among the stations based upon priority and position in a queue which is distributed across the buses, one for each direction. The DQDB has a slotted access protocol which uses a reverse bus to reserve a slot in order to make the access fairer. The fairness problems in DQDB networks and a proposed solution are discussed in a paper by E. L. Hahane, A. K. Choudbury, and N. F. Maxemchuk of AT&T entitled "Improving The Fairness of Distributed-Queue-Dual-Bus Networks" dated September, 1989.
In U.S. Pat. No. 4,922,244 to John L. Hullett et al. issued on May 1, 1990, there is disclosed a method of transmitting data on a communication network having two oppositely directed unidirectional buses (A, B) and a number of access units (4) coupled between the buses. The method includes the step of establishing a queue which is distributed in the access units and which controls when the access units can transmit data packets on the buses. When an access unit has a data packet (38) queued for transmission on the bus A, it sends a REQ bit on the bus B. The access units monitor the number of REQ bits passing and the packets available so as to thereby establish the distributed queue.
In U.S. Pat. No. 4,977,557 to Van P. T. Phung et al. issued on Dec. 11, 1990, there is disclosed a method of providing priority access in a communication system having dual unidirectional and oppositely oriented buses. An effectiveness factor K is introduced to enhance the effectiveness of priority. In one embodiment, lower priority countdown counters CD are incremented by K for each higher priority request. In a second embodiment, lower priority request counters RQ are also incremented. In a third embodiment, the low priority countdown counters CD slots are incremented only for the highest request received in a given slot. In a fourth embodiment, the lower priority request counters RQ are also incremented.
U.S. Pat. No. 5,001,707 to R. Kositpaiboon issued on Mar. 19, 1991, teaches a method of providing reserved bandwidth in a communication system having dual unidirectional and oppositely directed buses connected therebetween in which the bandwidth is provided by fixed format cells. The method includes the step of reserving a bandwidth on a data bus in response to requests accepted from stations on a signal bus. Each station is provided with a reserved bandwidth value which is calculated in dependence upon an amount of bandwidth requested by the station. A number of reserved bandwidth cells per a cell generation cycle is generated in dependence upon a total bandwidth requested by the stations. At each station, each reserved bandwidth used is counted until the reserved bandwidth value is reached.
However, none of the prior art discussed above discloses a method and apparatus for data transmission like that of the present invention which utilizes a medium access control (MAC) layer protocol in a multimedia slotted single bus local area network so as to achieve a full bandwidth utilization and a complete fair cell-access scheme that is independent of the number of network stations connected to the bus.